1. Technical Field
The present invention relates to a screw supercharger connected to an intake air pipe of an engine of an automobile or the like.
2. Background Art
In recent years, positive displacement screw superchargers are commonly used for automobiles.
The screw supercharger generally includes a male screw rotor and a female screw rotor engaged with each other, and these rotors are rotated by an engine to compress an intake air to be supplied to the engine.
When the engine does not need a compressed air (e.g., during a partial load condition in a particular transitional period from an idling condition to a constant speed condition) or when the supercharger is designed to suit for low speed condition but the engine is operated at a high speed condition, an excessive amount of air is supplied to the engine from the supercharger. If an excessive amount of air is supplied to the engine, a pressure ratio is raised and knocking likely occurs. Further, it causes lost motion or wasted work. Therefore, a flow rate of air to be supplied to the engine should be controlled.
Generally, a screw compressor for industrial use has a slide valve mechanism to adjust the flow rate of the supercharged air. However, the slide valve mechanism has a complicated structure and is expensive. In addition, the slide valve mechanism is not suited for a vehicle since a running condition of the vehicle changes significantly and quickly but the response of the slide valve is not prompt enough. Furthermore, it is difficult to insure decent longevity of sliding parts and associated parts of the valve mechanism.
In view of the above drawbacks, there is a proposal to provide a bypass line for returning the compressed air to the inlet of the screw supercharger from the exit of the screw supercharger. However, the air discharged from the supercharger has a high pressure and a high temperature. Thus, if the compressed air expelled from the exit of the supercharger is recirculated to the inlet of the supercharger, the air temperature at the supercharger inlet and in turn supercharger exit are accumulatedly raised by this recirculation. In this case, a certain measure should be taken to prevent knocking. For example, an intercooler should be provided or a compression ratio of the engine should be lowered.
However, providing the intercooler raises a manufacturing cost of the supercharger arrangement, and lowering the compression ratio of the engine results in deterioration of the engine performance.